1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and, more specifically, to high frequency, switched mode electronic power converters. The subject matter relates to improvement methods to achieve a reduction in the number of magnetic circuit elements in known power converters, reduction of electromagnetic interference (emi), and achievement of power converters with reduced size and cost.
2. Description of Related Art
In modern commercial power electronic circuits coupled magnetic circuit elements in the form of coupled inductors and transformers play a very basic role in converting power from one voltage to a different voltage. In many cases coupled magnetic circuit elements provide the galvanic isolation necessary for personal safety. Every coupled magnetic circuit elements has, inherent to it, a component of coupled or mutual inductance corresponding to shared magnetic flux between a pair of windings and a component of uncoupled or leakage inductance corresponding to magnetic flux generated by currents in a winding that is not shared by or coupled to the other winding. This situation is illustrated in FIG. 1(a) wherein some of the flux generated by winding 1 is coupled to winding 2 and some of the flux generated by winding 1 is not coupled to winding 2. FIG. 1(b) illustrates a circuit model of a coupled magnetic circuit element that places a “leakage inductor” in series with the primary winding. There are other workable models for coupled magnetic circuit elements that work about as well as the model illustrated in FIG. 1(b) and yield equivalent performance results. A circuit model that places a “leakage inductor” in series with the secondary winding instead of the primary winding or a circuit model that places “leakage inductors” in series with both primary and secondary windings yields results with no significant difference from the FIG. 1(b) model. The uncoupled or leakage inductance contributes to the self inductance of each winding but exists independently of the mutual inductance. The leakage inductance is to a large extent a controllable quantity, but it can never be entirely eliminated. In some cases leakage inductance has a beneficial effect. The cases in which leakage inductance provides a benefit are generally those cases in which the circuit is workable with no magnetic coupling. For all the cases in which the circuit cannot function without magnetic coupling, leakage inductance is, generally, a detriment. There are circuits that are unworkable without magnetic coupling which have been proposed in which the leakage inductance is claimed as a benefit because the leakage inductance is used to drive a zero voltage transition of a switch in the power converter circuit thereby reducing or eliminating first order switching losses and improving converter efficiency, but most, if not all, of these circuits ignore and seem unaware of the emi issues related to leakage inductance. In the circuits that rely on leakage inductance for an efficiency benefit the leakage inductance must be significantly increased to fully realize the benefit claimed, but increasing the leakage inductance in these circuits generally increases overshoot and ringing, which is related to the interaction of the leakage inductance and the circuit's capacitive parasitic circuit elements. In general, increasing the leakage inductance in circuits that are unworkable without mutual inductance creates an overall detrimental effect. The applicant is not aware of any practical commercial circuits that have used leakage inductance to achieve higher efficiency. The commercially practical circuits that, to date, achieve effective elimination of switching losses rely on a separate discrete inductor to provide energy to drive a zero voltage transition of a switch. These commercially practical circuits also contain a clamp diode to clamp ringing associated with the discrete inductor. An example of such a circuit is illustrated in FIG. 17. Since the leakage inductance does not have independently accessible terminals, a clamp diode cannot be readily provided to clamp ringing associated with the leakage inductance. The fact that the leakage inductance terminals are not fully accessible is one of the main reasons arguing in favor of using a discrete inductor rather than leakage inductance as the energy storage mechanism needed to drive a zero voltage transition. FIG. 2 illustrates a flyback converter that incorporates a primary side dissipative clamp and a secondary side snubber to deal with the potential ringing and overshoot problems created by leakage inductance. The circuits provided to deal with the leakage inductance are generally dissipative, but non-dissipative clamps and snubbers exist and can be used to eliminate emi. The non-dissipative clamps generally require a capacitor, a switch, and a drive circuit with control logic for the switch. The non-dissipative clamp circuits are now commercially practical because of the declining costs of semiconductor solutions and they are now in relatively wide use. Non-dissipative snubbers are not in wide use because they generally require additional costly magnetic circuit elements and they can be complex.
FIG. 3 provides an example of a circuit implementation that fails to fully solve the emi problem related to leakage inductance. The circuit is an active clamp flyback converter. The active clamp circuit is comprised of SPRICLAMP and CPRICLAMP. During an on state of the circuit, switch SPRIMAIN is in a conducting state (on) and switches SPRICLAMP and DSECMAIN are non-conducting (off). During the on state three of the four terminals of TMAIN are clamped to ac ground points in the circuit. The term ac ground is understood by those skilled in the art of power conversion to mean a circuit node wherein the voltage is invariant due to a direct connection to a ground or an indirect connection to a ground through a relatively large value capacitor or source of electromotive force (emf). At the power converter circuit's operating frequency the impedance of a relatively large value capacitor will appear as a short circuit. In the on state the primary winding is clamped, but the secondary winding is not clamped. During the on state of the FIG. 3 circuit, illustrated in FIG. 4, only terminal 3 of TMAIN has the potential to ring. Only terminal 3 of TMAIN is not clamped to an ac ground during the on state. Terminal 3, during the on state, is connected to a relatively small value capacitor associated with DSECMAIN that has the potential to oscillate and exchange energy with the leakage inductance through the mutual inductance of TMAIN. In actual practice, ringing and overshoot occurs at terminal 3 of TMAIN, creating an overvoltage stress and potential failure problem for DSECMAIN and an electromagnetic compatibility (emc) problem due to ringing at terminal 3 of TMAIN. The voltage and current wave forms associated with the FIG. 3 circuit are illustrated in FIGS. 6(a) through 6(g). During the off state of the FIG. 3 circuit, illustrated in FIG. 5, all four terminals of TMAIN are clamped to ac grounds. Both the primary winding and the secondary windings of TMAIN are clamped during the off state of the FIG. 3 circuit. Terminal 1 is connected to the positive terminal of CIN and to VIN in both operating states, so no voltage ringing can occur at terminal 1. Terminal 2 is clamped during the off state through SPRICLAMP to CPRICLAMP and through CPRICLAMP to VIN. In reality the leakage inductance energy rings with CPRICLAMP during the off state, but the ringing frequency, because of the large value of CPRICLAMP is very low, generally, much lower than the operating frequency of the power supply. The low frequency ringing associated with CPRICLAMP and the leakage inductance is also of very low amplitude. There are no adverse consequences to this low frequency ringing, so we shall ignore it, understanding that clamping leakage inductance energy through a large value capacitor eliminates adverse ringing, but allows a low frequency low amplitude ringing that has no adverse consequences. Terminal 3 is clamped to the secondary ground through DSECMAIN and terminal 4 is connected through COUT to the secondary ground. Since all four terminals of TMAIN are clamped during the off state, none of the four terminals can ring and, in fact, none of the terminals ring during the off state of the FIG. 3 circuit, although the leakage inductance energy is greatest at the onset of the off state. The point for the reader to remember is that no adverse ringing due to leakage inductance energy occurs when all four terminals of TMAIN are clamped. Whether the terminals of a winding are connected to ac grounds or not, a winding will not ring adversely if one terminal of the winding is connected through a low impedance conducting path to a relatively large value capacitor and the other terminal of the same winding is connected through a low impedance conducting path to the opposite terminal of the same capacitor. The connection of a capacitor across the winding forces the winding voltage to remain substantially invariant, which is what is meant when we say that the winding is clamped.
In the FIG. 2 primary circuit a dissipative clamp is provided consisting of DCLAMP, CCLAMP, and RCLAMP. At the beginning of the off state of the FIG. 2 circuit, leakage inductance energy is transferred through DCLAMP to CCLAMP and then the energy is dissipated in RCLAMP while discharging CCLAMP. In the FIG. 3 circuit the dissipative primary clamp of FIG. 2 is replaced by the active clamp comprising SPRICLAMP and CPRICLAMP. In the active clamp circuit energy from the leakage inductance is transferred to CPRICLAMP and then transferred back to the leakage inductance, where the energy can be used to drive a zero voltage turn on transition of SPRIMAIN. If there is insufficient energy in the leakage inductance to drive SPRIMAIN to zero volts, then the leakage inductance energy is useful for at least reducing the turn on switching losses of SPRIMAIN. If there is more than enough energy in the leakage inductance to fully drive SPRIMAIN to zero volts during the turn on transition of SPRIMAIN, then the excess energy, energy remaining after the zero voltage turn on transition is complete, is transferred to the secondary circuit and the load. In summary, the active clamp circuit is non-dissipative and provides an obvious benefit in terms of reduced switching losses in addition to its benefit of prevention of adverse ringing at terminal 2 of TMAIN.
What is needed is a way to eliminate emi associated with leakage inductance so that the leakage inductance can be used as a source of energy for zero voltage switching and improved efficiency.